ity between mothers and infants has not been described, and such similarity may not enhance the calls’ function. If similarity is not favored, there is
no reason to expect convergence between calls given by cross-fostered
infants and their mothers. This lack of functional consideration may have
interfered with progress in testing vocal learning in these and other primate
species.
The use of cross-fostering to test vocal learning requires differences in
vocalizations among species. Japanese and rhesus macaques differ in their
use of several calls. Japanese infants give coos during play almost exclusively, whereas rhesus infants use coos and gruffs (Fig. 4.3). Rather than
focusing on changes in acoustic structure, Owren et al. (1993) compare the
proportion of coos and gruffs given by cross-fostered infants to proportions
given by normally raised infants of each species. Cross-fostered Japanese
infants continue to give coos almost exclusively, in proportions indistinguishable from normally raised Japanese infants. Cross-fostered rhesus
infants give more coos than normally raised rhesus infants in two contexts
but fewer coos than normally raised Japanese infants in two other contexts
(Fig. 4.3B). Owren et al. (1993) interpret these results as very limited
modification of usage. Even when interacting with others producing gruffs,
cross-fostered Japanese infants almost never give these calls. Furthermore,
rhesus infants give gruffs even though they almost never hear their mothers
or social companions produce such calls. This result is particularly surprising because both species are capable of producing both coos and gruffs
(Owren et al. 1993), so very little modification is necessary to mimic the
calling behavior of social companions. New vocalizations do not need to be
learned. Perhaps modification does not occur because there is no strong
inducement to produce calls similar to those of social companions. Crossfostered infants interact normally with peers, despite their unusual vocal
behavior. Selection for call similarity seems to be lacking in these species
and may be overridden by other social cues such as facial displays and
gestures that facilitate social interactions. Thus, vocal learning may simply
not be necessary.
3. Group Signatures
Group signatures are signals that indicate an individual belongs to a particular social group. They are likely to be found in species where stable
social groups control access to limiting resources, such as food, mating and
nesting sites, or territories. In this context, discriminating between group
mates and others facilitates both cooperation within the group and exclusion of outsiders who compete for the same resources.Typically, signal structure is shared among group mates, and groups are distinct from one another,
although group members can learn to recognize each others’ individually
distinctive calls. Groups can share call types that are distinct from the types
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J.W. Boughman and C.F. Moss
no reason to expect convergence between calls given by cross-fostered
infants and their mothers. This lack of functional consideration may have
interfered with progress in testing vocal learning in these and other primate
species.
The use of cross-fostering to test vocal learning requires differences in
vocalizations among species. Japanese and rhesus macaques differ in their
use of several calls. Japanese infants give coos during play almost exclusively, whereas rhesus infants use coos and gruffs (Fig. 4.3). Rather than
focusing on changes in acoustic structure, Owren et al. (1993) compare the
proportion of coos and gruffs given by cross-fostered infants to proportions
given by normally raised infants of each species. Cross-fostered Japanese
infants continue to give coos almost exclusively, in proportions indistinguishable from normally raised Japanese infants. Cross-fostered rhesus
infants give more coos than normally raised rhesus infants in two contexts
but fewer coos than normally raised Japanese infants in two other contexts
(Fig. 4.3B). Owren et al. (1993) interpret these results as very limited
modification of usage. Even when interacting with others producing gruffs,
cross-fostered Japanese infants almost never give these calls. Furthermore,
rhesus infants give gruffs even though they almost never hear their mothers
or social companions produce such calls. This result is particularly surprising because both species are capable of producing both coos and gruffs
(Owren et al. 1993), so very little modification is necessary to mimic the
calling behavior of social companions. New vocalizations do not need to be
learned. Perhaps modification does not occur because there is no strong
inducement to produce calls similar to those of social companions. Crossfostered infants interact normally with peers, despite their unusual vocal
behavior. Selection for call similarity seems to be lacking in these species
and may be overridden by other social cues such as facial displays and
gestures that facilitate social interactions. Thus, vocal learning may simply
not be necessary.
3. Group Signatures
Group signatures are signals that indicate an individual belongs to a particular social group. They are likely to be found in species where stable
social groups control access to limiting resources, such as food, mating and
nesting sites, or territories. In this context, discriminating between group
mates and others facilitates both cooperation within the group and exclusion of outsiders who compete for the same resources.Typically, signal structure is shared among group mates, and groups are distinct from one another,
although group members can learn to recognize each others’ individually
distinctive calls. Groups can share call types that are distinct from the types
166
J.W. Boughman and C.F. Moss